A simple guide to GCHQ's internet surveillance programme Tempora

Revelations about the extent of the US government's ability to snoop on the private data of ordinary people have rocked the world.

The Prism programme, which former National Security Agency (NSA) contractor Edward Snowden leaked to the Guardian and

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Washington Post, has brought into relief the extent of surveillance in the digital age.

Now the Guardian, using documents leaked by Snowden, has laid bare the UK's surveillance capabilities, showing it to be an "intelligence superpower" that rivals even the US. Here's a short and concise roundup of everything you need to know.

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GCHQ, full name Government Communications Headquarters, is the UK spy agency dedicated to intelligence and information gathering.

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It's the UK equivalent of the NSA in the United States.

On 21 June the Guardian reported that GCHQ had placed data interceptors on fibre-optic cables that carry internet data in and out of the UK. These UK-based fibre optic cables include transatlantic cables that carry internet traffic between the US and Europe, meaning that GCHQ is able to directly access large amounts of global internet data. The programme is called Tempora.

They're taking data straight from the tubes?

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That's right. Interceptors have been placed on around 200 fibre optic cables where they come ashore. This appears to have been done with the secret co-operation, voluntary or forced, of the companies that operate the cables, potentially giving GCHQ access to 10 gigabits of data a second, or 21 petabytes a day.

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They use a technique called Massive Volume Reduction (MVR).

Peer-to-peer downloads, for example, are classed as "high-volume, low-value traffic" and discarded by an initial filter. This reduces the volume of data by 30 percent. They use specific searches, which can relate to trigger words, email addresses of interest, or targeted persons and phone numbers. GCHQ and the NSA have identified 40,000 and 30,000 triggers respectively.

So this isn't 'British eyes only'?

Nope. The data is shared with the NSA. In fact, 850,000 NSA contractors have access to the data, according to the documents reported on by the Guardian. It's possible that the UK and US intelligence agencies co-operate in order to bypass domestic restrictions on intelligence gathering -- the NSA isn't bound by UK restrictions on surveillance of UK citizens and GCHQ isn't bound by US restrictions on surveillance of US citizens.

How long has it been running?

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It was first trialled in 2008 and by the summer of 2011 GCHQ had placed interceptors on over 200 fibre optic cables. By late 2011, the Tempora programme had been fully launched and shared with the Americans on a three-month trial basis. The Americans, on their best behaviour, suitably impressed GCHQ and passed the test, reports the Guardian.

What's the legal justification for this programme?

Under the 2000 Regulation of Investigatory Powers Act (Ripa), defined targets can be tapped if there's a signed warrant. The warrant must be signed by the Home or Foreign Secretary. However, paragraph four of section eight of Ripa allows the Foreign Secretary to issue a certificate for broad interception of categories of material relating to terrorism or organised crime, for example. It appears that GCHQ is using that clause to justify the broad interception of web traffic.

GCHQ claims that its operatives behave within the law, including the Human Rights Act, which says that searches must be necessary and proportionate, meaning that there must be cause for looking at the data. GCHQ says that it doesn't snoop on ordinary citizens data, but targets bad guys like terrorists and criminals, and that the programme has prevented terrorist attacks on British soil.

How does this compare to what the NSA has been doing?

According to the documents, GCHQ's surveillance gives it the "biggest internet access" out of the "five eyes", which consist of spy agencies in Australia, New Zealand, the UK, Canada and the US.